With the expansion of the market in portable electronic devices such as mobile phones, laptop computers, and digital cameras, as a cordless power supply for these electronic devices, high energy density, long-lasting secondary batteries have been awaited. Then, in order to meet such demands, secondary batteries using lithium ions or other alkali metal ions as charged carriers, in which the electrochemical reaction accompanying the transfer of charge is utilized, have been developed. Among them, high energy density lithium ion secondary batteries have spread widely.
In such a lithium ion secondary battery, as an electrode material (mainly as a positive electrode active material), cobalt composite oxides having a layered rock-salt type crystal structure, such as lithium cobalt oxide, have been used. However, cobalt composite oxides have problems on the safety and are high-cost. Meanwhile, nickel composite oxides having the same structure as cobalt composite oxides, such as lithium nickel oxide, are safer than cobalt composite oxides and low-cost. However, they are difficult to synthesize and also need attention during storage. In addition, the crystal structure of nickel composite oxides is unstable. Therefore, when used as an electrode material for a battery, there are problems with safety.
In order to solve such problems, in recent years, researches have been actively conducted on the use of, as an electrode material, a lithium-nickel-manganese-cobalt composite oxide having the same structure as cobalt composite oxides and nickel composite oxides.
For example, JP-A-2003-238165 (hereinafter referred to as Patent Document 1) and JP-A-2003-31219 (hereinafter referred to as Patent Document 2) disclose an example in which a lithium-nickel-manganese-cobalt composite oxide is used as a positive electrode active material in a nonaqueous electrolyte secondary battery.
Incidentally, currently, lithium ion secondary batteries have been mounted on hybrid electric vehicles (HEV), electric cars (EV), and the like, and they have a wider range of applications. Accordingly, lithium ion secondary batteries are used in a wider variety of environments, and the maintenance of battery performance in high-temperature environments or low-temperature environments has also been one of important issues.    Patent Document 1: JP-A-2003-238165    Patent Document 2: JP-A-2003-31219